- The aim of the present study was to evaluate the relationship between serum progesterone level on the day of human chorionic gonadotropin (hCG) administration (day 0) and pregnancy rate among patients undergoing in vitro fertilization and embryo transfer. Serum progesterone levels were analyzed retrospectively in 160 cycles. In 56 women ovarian stimulation was induced by gonadotropins only (group A). In 104 cases a gonadotropin releasing hormone (GnRH) analog was given prior to gonadotropin administration (group B). At least three embryos were transferred in all cases. A significantly (p < 0.01) higher serum progesterone level (> or = 1 ng/ml) on day 0 was observed in the 43 patients of group A who did not conceive compared with the 13 who conceived. No significant difference in serum progesterone levels on day 0 was found between the 37 patients of group B who conceived and the 67 who did not. The pregnancy rate (0.36) in group B was significantly (p < 0.05) higher than that in group A (0.23). However, the pregnancy rate (0.35) in patients in group A with low progesterone levels (< or = 1 ng/ml) was not significantly lower than that in group B. We demonstrated that elevated progesterone levels (> or = 1 ng/ml) on day 0 in patients receiving stimulation with gonadotropin only, might be detrimental to pregnancy. In view of these results, we suggest that early oocyte retrieval should be considered in patients whose progesterone level on the day of hCG administration exceeds 1 ng/ml following stimulation with gonadotropin only.